[Partial Screen Sharing] Dismiss background after app selector launched

Finish the activity correctly

Fix: 283967615
Test: openPermissionActivity_launchAppSelector_activityIsFinished
Change-Id: I9110a43ee39671105ee16a64cd41ebc2a071237d

packages/SystemUI/src/com/android/systemui/media/MediaProjectionPermissionActivity.java

@@ -330,6 +330,8 @@ public class MediaProjectionPermissionActivity extends Activity
         // Don't send cancel if the user has moved on to the next activity.
         if (!mUserSelectingTask) {
             finish(RECORD_CANCEL, /* projection= */ null);
+        } else {
+            super.finish();
         }
     }